New Build: HF 79cc, Full Suspension, Extra-long MB

Discussion in '4-Stroke Engines' started by MikeJ, Jan 9, 2011.

  1. MikeJ

    MikeJ Member

    Hi Everyone -

    Here is my latest build (read: prototype) on which I have been spending a few hours here and there over the past few months. Photos should be attached.

    Comfort for long-haul bike rides was a major factor in this build. A previous build using a 2-cycle engine rode well on smooth surfaces, but I took a beating on rough roads. With this frame, I got balloon tires, shocks up front and in back, a better seat, and a less vibrating engine. There is almost no beating sent to the rider. The rear wheel frame pivots freely at the dropouts of the leading frame.

    I took frames of three bikes and combined them together. All three were steel frame mountain bikes: A 1994 Mtn Tek Extreme for the main frame and a Specialized Hardrock for the back part. The third bike contributed very little. All other parts are new out of necessity or desire:

    Front Rock Shox Dart 2 up front from a local bike dealer,
    79 cc 4-cycle engine from Harbor Freight,
    Extra-wide bottom bracket and drivetrain pieces from Sick Bike Parts,
    34 tooth MegaGear assembly from Sick Bike Parts,
    Derailler, chains, and wheels from a local bike dealer,
    Gas tank, throttle control and cables from Piston Bikes,
    Scooter shock from,
    Copper exhaust line and lots of hardware from Ace Hardware,
    Bearings from local industrial parts outlet,
    Engine platform and lots of JB Weld from Home Depot,
    Centrifugal belt clutch from MaxTorque,
    Multi-horsepower rated drive belt from Auto Zone.

    Yellow electrical tape covers a couple of places that I have to touch-up with the yellow paint.

    There was no additional hot welding done at all to this build. I can remove the back pivoting frame and rebuild a standard-length bicycle out of everything you see in the pictures.

    The bike rides nice without the engine and drive train. Today's test ride was with engine in place but without engine power. With an engine this wide, I will not pedal much; just enough to get it moving before engaging the clutch. That MegaGear will be a necessity when taking on long hills or pedaling without the engine. Going on flat land, gears 3 and 4 will be nice to have. I think it will ride like an underpowered early-60's Cadillac.

    The bike's absolute length is 84 inches; won't fit nicely in a pickup truck.
    The horizontal bar is 33 inches above the ground at back of gas tank,
    Tires are puncture resistant; 26 inches by 2.3 inches.
    Both wheels need disc brakes; a rear wheel disc brake assembly is on order.
    The engine platform is dense oak: 2.25" x 5" x 18". It is JB Welded to the frame and compressed together also by side-to-side bolts.
    Pillowblock bearings for the jackshaft are overkill, but they will not die.

    The bike as you see it (dry tank) weighs 87 pounds: 47 up front and 40 in back. (It must be the fast drying paint that is adding all that weight.) Weight creep is like middle age; Every extra part may add strength and functionality, but also an extra pound, or two, or four. . . .

    It still needs a kickstand, speedometer, tachometer, some lights that will be seen a half mile away, and a rack extending over the back wheel. There are only a few more parts to be installed. I already have a good, DOT-rated high-visibility motorcycle helmet, a bright yellow jacket and leather gloves.

    The pulley ratio is 3:1; the jackshaft to bottom bracket ration is 48:10. This works out to about a 15:1 drive train ratio. This is good; I computed that low-30 mph range will be little work for the engine.

    As big and heavy as this build is, I should park it in Compact Car Only spaces.

    I have more pictures of the in-progress build, but I figured a few here and more in the future will be welcomed. Let me know what you might have done to make this motorized bike better (like avoid excess weight).


    Attached Files:

  2. Rogwarf

    Rogwarf New Member

    How well does the chain do with the pivot? Nice clean looking build. I like the way the wood work came out.
  3. lowracer

    lowracer Member

    I really like what you've done here!
    Great Build...
    Before finding your thread, I was googling the Xtracycle & Surly Big Dummy. I was thinking of something similar using a donor bike attached at its bottom bracket to the main bikes' dropouts, but trying to figure out a way to mount my Lifan 97cc low like on the main bikes' chainstays?
  4. Wheres my dog

    Wheres my dog New Member

    VERY nice work and attention to detail...

    That HF 79cc looks tiny with the tank removed!
  5. ibdennyak

    ibdennyak Guest

    Ha.....sounds familiar, I went through the same process. I have a collection of parts to do just that. In my case, I make my own suspension triangles from scratch to get the length and height clearance, but using existing bushings etc. I have a couple of the same engines as you laying around, so that seems like a good power source. This is an upgrade of the stretch I have been riding the past year with the low mount engine. My only objection on that one is no rear suspension....but it certainly is stable and smooth.

    Will be starting on that as soon as I finish my front wheel drive with the Mitsubishi clone electric start and studded tires for winter using the double rim drive idea that I am stealing from you. :devilish:
  6. ibdennyak

    ibdennyak Guest

    Nice looking bike Mike. When I read about the wood engine mounts I wondered, but after seeing it.....I like it. :bowdown:
  7. professor

    professor Active Member

    Great job and great thinking.
    After richening up the main jet a tad on my HF, the choke can be turned off after a couple of minutes (rather than many minutes) it has been a quality engine.

    To me, your bike is not heavy. Long for sure though.

    I have been thinking of changing the frame in back of mine by welding in a rear traiangle from a suspension bike and re-working the frame, but would like to keep the separate right side drive chain. I like to be able to pedal with the engine up to 20 mph or so. Are you able to do that?
  8. lowracer

    lowracer Member

    I was thinking of using my whizzer sheave & max torque pulley clutch directly on the left side & keep the right side for pedaling.
    Since my pusher trailer was a flop (no pun intended) & my rear rack lifan is great up to a certain speed, I'm thinking the added wheelbase would aid handling at speed?
  9. professor

    professor Active Member

    Here is the trade off- long wheelbase= hi speed stability/ low speed sluggishness in turning. Opposite for short wheelbase.
  10. lowracer

    lowracer Member

    You got my wheels turning.
    Did you bolt the rear dropouts from the front section directly to a bottom bracket from the rear section?
    Any spacers required or a direct bolt together?
    How is the torsional rigidity between sections?
  11. MikeJ

    MikeJ Member

    Hi All -

    Thanks for your comments. Here are a couple of answers: The chain and the pivot... I suspect that if going over bumps during a hard pull, the chain will be pulled really tight when the wheel returns downward. To avoid snapping the chain or having the back wheel pulled out of place, I may have to place a spring loaded tension arm on the pulling side of the chain to lift it up and provide slack to the wheel after coming off the bump. I don't know yet what will work; probably a piece of steel with a little curvature in it to provide chain lift, yet allow the chain to slide sideways when changing gears. I may provide a surface of plastic like a half-moon-shaped cutting board for the chain to slide over during operation. Will it work? I have to try before judging. If this proves to be a real problem, I will simply remove the shock absorber and substitute the proper length of electrical conduit or steel water pipe; very easy to do.

    The pivoting shaft took quite a bit of work. In a quick summary: I cut away the bottom bracket housing of a third steel bike(from discard pile), split it, and added the halves to widen the bottom bracket housing of the second donor bike to just fit between the dropouts of the Mtn Tek bike. A 3/8 inch diameter bolt (8 inches long) from Ace Hardware and some thick wall spacers and washers (3/8 inch internal diameter, 5/8 inch outside diameter) act as a solid shaft for inexpensive 5/8 inch internal diameter bearings that just barely fit in the lengthened bracket housing when the nut on the end of the bolt is tightened down. (Use a Dremel grinding wheel to open the internal diameter of the three-piece housing; it does not take too much effort. I used MEK solvent and JB Weld on the bearing and housing surfaces to seal the bearings into place. Don't get sloppy with JB Weld!) The 3/8 inch diameter bolt just fits where the bicycle shaft fits. For max housing strength, I recommend hot welding the bracket housing assembly. The installed bearings can slide back and forth maybe half a millimeter on the compressed shaft while between the dropouts. This ensures the dropouts don't pinch the bearings when turning fairly hard on the nut of the axle. The additional housing width eliminates wobble when the wheel is rotating. The extending frame should pivot easily and not display signs of binding.

    Long-winded explanation? Yes. Hard to envision? It sure is. But this assembly is not found at any bike shop, infomercial, or anyplace else. Each one will be as unique as they can be.


    Attached Files:

  12. lowracer

    lowracer Member

    Thanks for the description & pic.
    Your machine is quite an accomplishment!
  13. MikeJ

    MikeJ Member

    Hi Everybody -

    I started building my bike a few months back, and finally started testing it during the past few days. I took out the rear spring that I displayed in earlier photos and placed in a solid tube. During the first day of testing, I had a chain problem... The extra long drive chain was "bouncing" up and down during acceleration, causing problems with handling.

    I solved my problem. Recall my bike's long drive chain was "bouncing", causing my bike to "pulse" forward. I figured that if I could stop the bouncing, the pulsing would stop. Here is my fix:

    Went to Ace Hardware (spent LOTS of money there). I purchased a length of their large diameter plastic (silicon?) tube. See photos. I also purchased a 5/8 inch solid shaft, eight inches long. They had 5/8 inch diameter bolts for less than $3 each (Great! No cutting involved!). I already had the inexpensive Ace roller bearings of 5/8 inch internal diameter, which just happen to fit perfectly inside the plastic tube! Nice fit! The shaft rested nicely in the frame valley, the 2 inch length of tube rotated without friction, the chain rested on top of the relatively soft plastic, and placed a very slight apex angle in the chain when it was under tension. I fastened it all down with intent to make it easy to remove if necessary. So visually, it is not real pretty. But it works! (You may need to rotate the second photo; I had to upload it as you see it.)

    During an 8-mile test ride late today, the chain rode over this "chain idler". The idler rotated at chain speed, and made not a sound whatsoever. And accelerate as hard as I tried, there was no chain bounce! Hooray! Another engineering problem solved! With readily available parts!

    Now about that 79 cc HF engine... In the 2000 to 2400 rpm range, that little engine likes to produce some serious torque pulses during its power stroke. Given the length of my bike frame and its inherent flexing, my bike does quite a bit of shaking until the engine gets over 2400 rpm. Then the bike settles down and rides as expected. I noted some other unexpected additional random shaking. Then I realized that was from the imperfect road surface.

    At my altitude (currently in Albuquerque) of 5500 feet, I looked up derating an engine because of high altitude. At full throttle, gasoline engines are derated. One chart I found derated at 3 percent for every 1,000 feet above sea level. So I am at a slight disadvantage compared to sea-level bikes in all-out acceleration and wide-open throttle top end.

    I added a cargo basket and some rear attention-getting lights. The brake light is wired to go full bright when I squeeze the rear brake handle; it is an LED truck light powered by a 12 volt sealed lead acid battery. The strobe flasher (from Radio Shack about 16 years ago) is a real attention getter in the dark. I safety-wired it down so it will not break loose if I hit a nasty pothole. Now I just need some headlights, red reflectors, and the air horn that I already have. Then I am ready for night riding!

    Given that long exhaust pipe, I don't think a lawnmower muffler would do much more.... I think it runs reasonably quiet as it is. There is certainly NO chain noise!

    The bike is in its break-in phase right now. It has only 18 miles on it. Nothing has broken (yet). It needs more stress testing. In a couple of weeks, I may take it to the east to a town about 30 miles from here. There is a 7,000 foot pass between here and there. I think the bike and I will make it if I don't overrev the engine.

    That is my long-winded story. I will probably have more to write in the future....


    Attached Files:

  14. lowracer

    lowracer Member

    Lookin good.
    Not sure how long that silicone tubing roller will last? You may want to look into Terratrike idlers if it tears up quickly. They are very popular among the recumbent bike crowd & work well.
  15. BikeMan

    BikeMan Member


    Hope you don't mind I messaged you with a question.Thanks.
  16. Cheap chain tensioner!

    I built a recumbent from an old BMX frame. I needed a chain tensioner so I got a muffler (U bolt) clamp and an old derailleur. I just put the U bolt where I thought it should go on the frame then bolted the derailleur to the lower leg of the U bolt using washers and an extra Nylock nut. Ran the chain through there and worked excellent! Cost: less than $2.00 for the muffler clamp! Wish I had a photo of it, but I think it is pretty straight forward. A Huffy MTB derailleur has a longer cage so it allows for more chain take up. (Gotta love those low end Huffys! :bowdown:).
    Last edited: Apr 15, 2011
  17. I also used a U bolt and an in-line skate wheel on one leg of the bolt as a chain idler. I put the wheel on a bolt and tightened it so it could not spin. I put a round file in a vice, chucked the bolt in a drill and held the wheel against the file until I got the groove I needed to keep the chain in place. Bolted the wheel to the U bolt. Worked very well!